Conventionally, after all thin film layers on a multichip module (MCM) have been fabricated, a full electrical test is performed to confirm the integrity of the completed wiring. If any defect is detected at this stage, an after-thin-film (ATF) repair is performed to correct the defective nets.
FIG. 1 shows a plan view of a typical MCM 100. In FIG. 1, chips 102, 104, 106, 108, 110, 112, and 114 are mounted to the top surface metallurgy (TSM) of MCM 100 using a Controlled-Collapsed-Chip-Connection (C4) configuration (not shown in this Figure). Seven chip locations are shown in FIG. 1. MCMs are not limited to this configuration, however, and may be any number of chips depending on the requirements of the application. Before mounting the chips 102 through 114, MCM 100 is tested to ensure that no wiring defect such as an open or a short exists in MCM 100. If a wiring defect is found, the MCM must be repaired. Shorts between power planes are a particularly difficult problem in packaging both from a yield perspective and from a diagnostic perspective. Tests for power plane shorts are relatively simple, yet such shorts are difficult to locate and are an important class of critical defects.
The conventional ATF repair strategy discards the entire original net wiring and reconstructs new net wiring using the top surface repair lines, modifying their lengths to match the required electrical properties of the deleted wiring net. This conventional ATF repair method had worked well for traditional thin-film MCM manufacturing. For close tolerance thin-film MCM products, however, a drawback of this conventional repair process is that product yield is adversely affected if the number of nets requiring repair exceeds the number of available repair nets on the TSM.
Referring again to FIG. 1, a typical pair of wiring nets 116, 118 is shown. For illustrative purposes, it is assumed that a short circuit exists between wiring nets 116, 118. The conventional repair process deletes all of wiring nets 116, 118 by cutting wiring nets 116, 118 at C4 location 120. In this example, wiring nets 116, 118 are cut (also called deletes) at sites 122, 124, 126, 128, 130, 132, 134, 136, 138, 140, 142, 144, 146, and 148. The deleted wiring nets 116, 118 must be replaced using the TSM repair net (shown in FIG. 2A).
FIGS. 2A and 2B show a typical TSM repair net 200 for the MCM of FIG. 1. In FIG. 2A, repair net 200 has x-lines 202 and y-lines 204. As shown in FIG. 2B, within the gridwork of repair net 200 are C4 connections 206 for each chip 102, 104, 106, 108, 110, 112, and 114 mounted on MCM 100.
FIG. 2C shows an x-ray view of a five-layer MCM and FIG. 2D is a partial side view of MCM 100 illustrating the layered structure of MCM 100. In FIG. 2C, successive layers form MCM 100. Typical layers include ground layer 208, power layer 210, x-layer 212, and y-layer 214. An additional layer, top layer 216 (shown in FIG. 2D), contains repair net 200 and C4 connections 206. It is apparent from FIG. 2C that repair of an internal short circuit between any two x-layer lines or y-layer lines is a formidable task. For this reason, conventional repair processes deleted defective nets at the top layer 216.
As mentioned above, conventional ATF repair is based on full repair. That is, the entire internal structure of a defective net is removed at its C4 connections 206. An entirely new set of wiring is reconstructed using repair net 200 and connected to the C4 connections 206 on the TSM. These full repairs are expensive and time consuming. Full repairs are often necessary, however, because frequently the location of the defect in the defective net is unclear and the construction of a new net is the only practical way to repair the defective net.
Generally, power-to-power shorts or defects such as opens or shorts on critical input-output (IO) nets, such as IO nets connected to a BSM (bottom side metallurgy) IO pad which will be connected to a BSM pin, must be repaired at or near the wiring level manufactured. Otherwise, the part will be scrapped when the defect is discovered later at a more costly level of build when repair may no longer be possible or where the defect may no longer be visible due to layers of metal and insulator above it.
In the case of multilayer thin-film MCMS, the best opportunity to diagnose, locate, and repair defects in the thin-film structures occurs after each metal layer is added. The extraordinarily complex configuration of the thin film structures and the high topology render it difficult, however, to pinpoint defects such as shorts even when the approximate location of the short is known.
Historically, infrared (IR) techniques have been used to locate power plane shorts on electronic packaging devices such as printed circuit boards. A current is directed through the short, heating the area around it, and an IR camera is used to approximate the location. These techniques may be destructive and often require special optics not readily available or accessible on the manufacturing floor. In addition, the device may provide a heat sink making it difficult to obtain a significant difference in temperature between the short and the background.
Automated optical inspection (AOI) techniques provide another approach used to locate power plane shorts. AOI techniques require large, relatively expensive inspection devices. In addition, although such techniques work well for many classes of defects, there are always some classes of defects which escape optical detection. Optical techniques will tend to be particularly poor for unusual defects such as inter-level shorts (ILS) not clearly visible from the top of the device. Manual inspection on extremely complex products may likewise be ineffective.
A more recent alternative is to pass a high-frequency current through the defect and approximate the location of the short using a magnetic pickup coil apparatus. Such magnetic induction methods are generally limited in spatial resolution and only approximate the general location of the defect. Further apparatus may be necessary to precisely pinpoint the location of the defect. In addition, magnetic induction techniques assume that an operator can recognize a short once its location is observed under a microscope. There remains a need, therefore, for a technique which can pinpoint the location of a defect in a small area of a device and can verify whether a suspicious point is, in fact, a defect.
Yet another alternative exists for locating power plane shorts. Once all other alternatives are exhausted, if the location of the short remains unknown, a sufficiently high current can be applied to blow the short. This high-current stress technique may sometimes be helpful. A category of shorts will still escape detection, however, using any or all of the conventional techniques discussed above. This category of shorts would include shorts which are large enough not to easily be blown by a current stress yet difficult to recognize visually. One possible example of a short in this category is an interlevel short which shorts two power planes vertically yet cannot be easily confirmed as a short visually with any certainty. To assure better yields and diagnostics, therefore, it would be desirable to provide an improved technique which can find this category of shorts.
Finally, a problem related to the task of locating power plane shorts exists: it is regularly necessary to determine whether or not a suspicious-looking defect is a short. The conventional techniques often cannot discern whether a particular defect shorts one layer of thin film to another. It would be desirable to verify whether these defect are in fact voltage plane shorts before attempting to repair them.
The deficiencies of the conventional techniques (e.g., IR imaging, magnetic imaging, pick-up coil imaging, current stress) used to locate power plane shorts show that a need still exists for an improved technique. That need is for a practical and convenient technique which can be used to detect, locate, and define inter-level shorts or otherwise difficult-to-recognize shorts. To meet that need and to overcome the shortcomings of the conventional techniques, a new apparatus and method to achieve these functions is provided.
An object of the present invention is to provide a practical and convenient technique for detecting, locating, and defining inter-level shorts or otherwise difficult-to-recognize shorts. A related object is to provide a practical and cost-effective apparatus and method to achieve these functions. Another object is to provide a convenient apparatus and method which integrate with existing quality control devices and processes.